This invention relates to sandwich panel composite structures comprising fiber reinforced low density closed cell material, fibrous skin reinforcements and resin, and in particular to improved structural configurations, improved methods of resin infusion and methods of production.
Structural sandwich panels having cores comprised of low density closed cell material, such as plastic closed cell foam, and opposing skins comprised of fibrous reinforcing mats or fabrics in a matrix of cured resin have been used for many decades in the construction of a wide variety of products, for example, boat hulls and refrigerated trailers. The foam core serves to separate and stabilize the structural skins, resist shear and compressive loads, and provide thermal insulation.
The structural performance of sandwich panels having foam cores may be markedly enhanced by providing a structure of fibrous reinforcing members within the foam core to both strengthen the core and improve attachment of the core to the panel skins, for example, as disclosed in Applicant""s U.S. Pat. No. 5,834,082. When porous and fibrous reinforcements are introduced into the closed cell foam core and a porous and fibrous skin reinforcing fabric or mat is applied to each face of the core, adhesive resin, such as polyester, vinyl ester or epoxy, may be flowed throughout all of the porous skin and core reinforcements by differential pressure, for example under a vacuum bag. While impregnating the fibrous reinforcements, resin does not saturate the plastic foam core because of its closed cell composition. The resin then co-cures throughout the reinforced structure to provide a strong monolithic panel.
It is desirable to produce sandwich panels of enhanced structural performance by improving the structural connections and support among reinforcing members within the foam core and between the core and the panel skins. This is desirable in order to resist buckling loads in the reinforcing members, to prevent premature detachment of reinforcing members from one another and from the skins under load, and to provide multiple load paths for the distribution of forces applied to the panel.
Existing fiber reinforced core products offer important improvements over unreinforced foam in this regard but fail to integrate fully the separate reinforcing elements of the core into a unified and internally supported structure. For example, in a grid-like configuration of fibrous reinforcing sheet-type webs in which a first set of continuous webs is intersected by a second set of interrupted or discontinuous webs, the webs do support each other against buckling. Thus, under severe loading conditions, the discontinuous webs tend to fail at the adhesive resin bond to the continuous webs along their narrow line of intersection. This tendency may be substantially reduced by providing resin filled fillet grooves in the foam along the lines of intersection as disclosed in the above mentioned patent. However, since the reinforcing fibers of interrupted webs terminate at each intersection with a continuous web, the structural contribution of those fibers is substantially less than that of the fibers of the continuous webs.
In the case of strut or rod type core reinforcements comprising rovings of fiberglass or carbon fiber or other fibers which extend between the faces of the core, individual struts within a given row of struts may intersect each other in a lattice configuration. This supplies buckling support to each strut, but only in the plane of the strut row. To achieve bidirectional support, struts of a first row must extend through the filaments of struts of an intersecting row. This requires difficult and costly levels of accuracy and control in machine processing, since all struts must be precisely positioned in three dimensions.
One embodiment of the present invention overcomes the limitations of both web type and strut type reinforced foam cores by combining these two types of reinforcing elements into hybrid reinforcement configurations. In hybrid architecture the foam core is provided with parallel spaced rows of fibrous reinforcing webs or sheets which extend between the faces of the foam board at an acute or right angle. A second set of parallel spaced rows of reinforcing elements comprising rod-like fibrous rovings or struts also extend between the faces of the foam board at acute or right angles, and the rovings or struts intersect the webs and extend through them. Thus webs and struts constitute an interlocking three dimensional support structure in which all reinforcing fibers within the core are uninterrupted. The interconnected webs and struts provide multiple load paths to distribute normal loads efficiently among the reinforcing elements of the core and between the core structure and the panel skins. Impact damage tends to be limited to the immediate area of impact, since the complex reinforcement structure resists the development of shear planes within the core.
In an alternate hybrid architecture, the webs comprise a continuous sheet of fabric or mat which is formed into corrugations having segments which extend between the faces of the core, and the voids between the corrugations are filled with foam strips of matching cross-section. The corrugations, together with the intersecting panel skins, may form, in cross-section, rectangles, triangles, parallelograms or other geometric shapes which are structurally efficient or which offer manufacturing advantages.
In a particularly cost efficient version of hybrid core, the core reinforcing webs are produced by winding relatively low cost fibrous rovings in a helical manner onto rectangular foam strips, rather than by adhering substantially more expensive woven or stitched fabric to the surface of the foam strips. Additional rovings may be applied axially along the length of the strips during the winding operation to enhance structural properties of the strips or to serve as low cost components of the finished panel skins. The fiber-wound foam strips may also be attached together to form a structural core without the addition of rows of structural struts. In this configuration, the contiguous or adjacent sides of wound strips of rectangular cross section form web elements having I-beam flanges for attachment to panel skins. In contrast to the disclosure of U.S. Pat. No. 4,411,939, the fibrous extensions of each core web are attached to panel skins on both sides of the web rather than only one, greatly increasing the shear strength of the resulting panel. This permits the use of lighter and less expensive webs for a given strength requirement. Similarly, the present invention provides markedly improved core-to-skin attachment and shear strength when compared to the structure disclosed in Applicant""s U.S. Pat. Nos. 5,462,623, 5,589,243 and 5,834,082. In tests, webs comprised of circumferentially wound rovings exhibit 75% greater shear strength than those whose end portions terminate adjacent the panel skins. Each wound strip may be provided with internal transverse reinforcing webs to provide bi-directional strength and stiffness. Roving-wound cores may also be formed using strips of triangular cross section.
The winding of rovings by machine and the consolidation of the fiber-wound strips into a single core have both economic and handling advantages. It is common for a single composite bridge deck panel or yacht hull constructed in accordance with U. S. Pat. No. 5,701,234, 5,904,972 or 5,958,325 to comprise a thousand or more individual core blocks. The labor component of producing these individual cores is very high. Reinforcement fabric is cut into sheets which are wrapped and glued around each separate core, or smaller pieces of fabric are glued to the separate faces of each core, or tubular fabrics are first formed and the cores inserted into them. These processes become increasingly difficult as the dimensions of the core components decrease. Arrangement of these cores in a mold is also labor intensive, expensive and time consuming, which restricts the number of panels which may be produced from a mold in a given period of time. Positioning of individual core blocks becomes increasingly awkward as the curvature of the mold increases or as the mold surface departs from horizontal. The cores which are the subject of the present invention substantially eliminate these deficiencies by unitizing a large number of components into a single, easily handled core.
In addition to their superior structural performance, hybrid design allows economical production of extremely complex and structurally efficient configurations through relatively simple processes at high machine throughput and without requiring extreme levels of manufacturing precision. As mentioned above, bidirectional strut type cores require the insertion of roving reinforcements into the foam board with a degree of accuracy which is difficult to achieve when it is desired that rovings of intersecting rows extend through one another. It is also necessary to make multiple passes through strut insertion devices in order to place struts angled in two to four directions within the board.
In contrast, bidirectional hybrid cores may be produced in as little as a single pass through a strut insertion device. The reinforcement webs cooperate with the intersecting struts to resist loads in the plane of the struts. The webs also provide strength in the direction transverse to the struts, since the webs extend transversely to the rows of struts. Further, a much more limited degree of accuracy is required in production, since the struts have only to intersect the plane of the webs, rather than a narrow bundle of filaments.
Hybrid cores improve production of molded panels by increasing the rate and reliability of resin impregnation or infusion of both the core reinforcing elements and the sandwich panel skins which overlie the core. In vacuum assisted resin transfer molding (VARTM) processes, panels comprising dry and porous skin reinforcements are placed in a closed mold or a single sided mold in which the panel is covered by a sealed bag impermeable to air. The panel is then evacuated, and resin under atmospheric pressure is allowed to flow into and infuse the reinforcements. Because of the complex interconnections between the webs and struts in the cores of the present invention, both air and resin are able to flow rapidly and pervasively throughout the structure. The porous webs and struts form natural resin flow paths between the skins and carry resin rapidly from its source of introduction to a multiplicity of points at the porous skins. This minimizes the problem of race tracking, in which areas of dry skin fabric become isolated from the vacuum source by an unevenly advancing resin front, preventing the skins to wet out fully before the resin begins to thicken and cure.
In one embodiment of the present invention, no resin distribution medium of any kind is required between the panel skins and the mold surface or vacuum bag membrane. This not only eliminates the cost of such distribution medium but also allows the production of panels having smooth faces on all sides. Also, in contrast with prior art such as disclosed in U.S. Pat. No. 5,958,325, the foam core need not be provided with micro grooves located on the periphery of the core adjacent the panel skins, or with slots or holes in the foam which extend between the skins, as the means for distributing resin to the skins. In the present invention, all resin flows to the skins through the core reinforcing structure, whereas U.S. Pat. No. 5,958,325 specifically describes impregnation as resulting from resin infusion originating at the core surface. A disadvantage of peripheral micro grooves is that the size and spacing of the micro grooves must be selected to match the type and quantity of the panel""s fibrous fabrics in order to insure full impregnation of the skin and core reinforcements before the resin cures. In the present invention, all of the resin which infuses the skins passes through the porous reinforcing structure of the core to reach the skins, and since panel skins are typically intersected by two or more porous reinforcing elements per square inch of panel surface, resin tends to spread both rapidly and evenly across the skin surface. Thorough impregnation of the panel skins, which can be seen, is a reliable indicator that the core reinforcing structure does not have dry, and therefore weak areas. This is an important advantage over other infusion systems, in which resin is introduced adjacent the skins.
In accordance with the present invention, resin is supplied to the core reinforcing structure through a network of grooves within the interior of the foam core and adjacent the core reinforcing webs and extending parallel to the webs, and not adjacent the panel skins. The ends of these grooves intersect feeder channels which usually have a larger cross-sectional area. Resin supplied to the feeder channels rapidly flows through the grooves adjacent the webs and substantially all of the resin then flows through the fibrous core reinforcing elements to reach and impregnate the panel skins. If the resin grooves are located in a plane near the center of the panel thickness, resin need only flow through half the thickness of the panel, in each direction from the center plane, before full resin saturation is achieved. This is markedly faster than common resin infusion techniques in which resin is introduced across a single panel face and must flow through the entire panel thickness to reach and infuse the opposing face. Panels with thick cores or thick skins may be provided with one or more additional sets of resin grooves and feeder channels for faster infusion. The sets of grooves and feeder channels describe a plurality of planes parallel to the panel faces.
The infusion method of the present invention is particularly well suited for the production of molded panels in which both faces of the panel require a superior surface finish. Because resin is introduced into the interior of the core and flows rapidly under differential pressure throughout the core to the skin reinforcing structure, both faces of the panel may be adjacent rigid mold surfaces of desired shape and finish, without seriously increasing the time required for infusion compared to infusion conducted under a flexible surface, such as a vacuum bag. In contrast, common differential pressure molding processes such as VARTM, in which the skin reinforcements are consolidated by pressure prior to the introduction of resin, require that one side of the panel be covered with a flexible membrane, such as a vacuum bag, enclosing a resin distribution medium if it is desired to both maintain substantial pressure and introduce resin rapidly over the skin surface. If this arrangement is not used, the pressure of rigid mold surfaces against both panel faces necessitates a long and slow infusion path, in which the resin impregnates the skins by flowing along their length and width, rather than through their thickness.
The inside-out core infusion method of the invention may be used to infuse into the fibrous core reinforcements and inner skin layers a resin which differs in properties from the resin which infuses the outer skin layers of the panel. It may be used, for example, to produce a sandwich panel having an outer skin layer comprising fire resistant phenolic resin and an inner skin layer and core reinforcement structure comprising structural vinyl ester resin. This is achieved by providing an adhesive barrier, for example of epoxy resin in film form, between inner and outer layers of porous, fibrous skin reinforcements. A first resin is supplied by infusion from within the core as previously described, and a second resin is infused directly into the outer skin reinforcements, with the barrier film serving to keep the resins separate while creating a structural adhesive bond between them.
In a useful variation of the hybrid core of the invention, the reinforcing webs do not extend between the faces of the panel. Instead, two or more foam boards are interleaved with porous, fibrous web sheets and stacked in a sandwich configuration. Porous roving struts or rods extend between the faces of the core and through the intermediate web sheet or sheets. The web or webs stabilize the struts against buckling under load and also serve to distribute resin to the struts and skins. Resin may be introduced through parallel spaced grooves in the foam adjacent the web. Alternately, resin may be flowed into the core through a feeder channel which is perpendicular to the panel faces and which terminates in radial grooves adjacent the webs. This arrangement is useful in infusing circular panels, for example, manhole covers. In a third variation, the web sheet may incorporate low density fibrous mat or non-structural, porous infusion medium through which resin supplied through feeder channels flows across the center plane of the panel to the struts and through the struts to the panel skins.
An additional feature of the present invention is the provision of improved connections between strut or rod-type core reinforcing elements and sandwich panel skins. This improvement is applicable to hybrid panels having both web and strut-type core reinforcing members, as well as to panels whose core reinforcing comprises only struts. The porous and fibrous struts which extend between the faces of the core may terminate between the core and the skins, may extend through the skins and terminate at their exterior surfaces, or may overlie one or more layers of the panel skins. Under load, the struts are subject to substantial forces of tension or compression at the point of intersection with the skins, and these forces may cause failure of the adhesive bond between reinforcing element and skins.
Prior art, for example, as disclosed in European Patent No. 0 672,805 B1, discloses the provision of looped end portions of the reinforcing elements adjacent the skins. Under pressure during molding, the loops formed in the end portions of the struts provide an expanded area of adhesive contact with the skins. However, a serious disadvantage of this design is that the loops, which are doubled-back bundles of fibers, form lumps which cause the panel skins to deform out of plane under molding pressure. This results in excess resin accumulation in the skins, an increase in the tendency of the skin to buckle under in-plane compressive loads, and undesirable surface finishes.
In the present invention, terminating ends of strut type reinforcing elements are cut to allow the filaments which comprise the struts to flare laterally under molding pressure, which both significantly flattens the end portions against the skins and provides an expanded area of adhesive bond between each strut end portion and skin in the region immediately adjacent the strut end portion. Skin surface flatness may be further improved by applying sufficient pressure, sometimes simultaneous with heat, to the faces of the panel before molding to provide recesses for embedding any reinforcement lumps or ridges into the foam core during the molding process. Alternately, grooves may be formed in the faces of the foam along the lines of strut insertion, into which strut end portions or overlying stitch portions are pressed during molding.
The present invention also provides an alternate method of anchoring strut ends and which is effective even when the strut end portions do not overlie panel skins. In this configuration, parallel grooves or slits are so located in the faces of the foam board that the end portions of strut-type reinforcing members pass through the grooves. Porous reinforcing rovings having sufficient depth to adhesively anchor the strut ends are inserted into the grooves prior to insertion of the strut members, and resin which flows into the structure during molding provides structural attachment of struts to the rovings within the grooves. The rovings, having a substantial area of contact with the overlying panel skins complete the transfer of structural loads between skins and cores. An important additional benefit of this construction is that the groove rovings and strut members may be sized so as to constitute a unitized truss structure, with the groove rovings serving as truss chords. Since rovings cost substantially less than woven fabrics, this allows for economical panel fabrication in cases where relatively thin skins are adequate between the truss rows.
In the present invention, low cost rovings may also be applied directly to the faces of the foam boards to form panel skins during the process of inserting reinforcing members into the foam and in lieu of applying skins of more costly woven or knitted fabric reinforcements to the faces of the core. In this method, multiple rovings are supplied along parallel lines transverse to the core length and are drawn in a longitudinal direction continuously from supply creels by the forward progress of the foam core through the strut insertion machine, in sufficient number to more or less cover the faces of the foam. Prior to strut insertion, groups of rovings are drawn transversely, at right or acute angles, across the faces of the core from creels and advance with the core while strut rovings are stitched through the core. Overlying portions of the stitches hold all surface rovings in position to form a structural panel skin once resin has been applied to the panel. If desired, a light veil of reinforcing material may be applied over the surface rovings before stitching to improve the handling characteristics of the core prior to molding. In lieu of continuous rovings, random or oriented chopped rovings may be applied between the core faces and surface veils to form a structural mat.